Apple announced today that iPhone 4S sales topped 4 million in the device’s first 3 days of availability. “iPhone 4S is off to a great start with more than four million sold in its first weekend—the most ever for a phone and more than double the iPhone 4 launch during its first three days,” said Philip Schiller, Apple’s SVP of Worldwide Product Marketing. “iPhone 4S is a hit with customers around the world, and together with iOS 5 and iCloud, is the best iPhone ever.”

The device is selling so well, that if you want to get it at the Apple Retail Store, you’ll have to reserve one. If you go to Apple’s “how to buy” page for the iPhone 4S, it now presents you with two options (hat tip to MacRumors): “Buy now” at the Apple Online Store or “Reserve” at the Apple Retail Store.

With the second option, the message says, “iPhone 4S is available in store by reservation only. Reserve your safter 9:00 p.m. tonight for pickup tomorrow. When you come in, you’ll choose a carrier and plan, and we’ll get your iPhone up and running before you leave the store.”

Once you click on the button to reserve, you are prompted to choose a store to pick up the iPhone. You’re also presented with another message that says, “iPhone 4S is available at the Apple Store by reservation on a first-come, first-served basis.”

Once you choose a store, you’re presented with 18 options: The 16, 32, and 64GB versions in white or black, for AT&T, Sprint or Verizon. Here, you see that for the store I selected, they’re all marked as “unavailable”:

Presumably, that will change after 9:00 PM.